1. Field of the Invention
This invention concerns an ionization vacuum gauge, and in particular a hybrid ionization vacuum gauge which incorporates another measurement part into an ionization vacuum gauge.
2. Description of Related Art
In various conventional semiconductor manufacturing systems and electronic device manufacturing systems which employ high vacuums, upon system startup, during maintenance, and as conditions for various processes, pressure measurements must be performed over a wide range ranging from atmospheric pressure to high vacuum regions. Diverse vacuum gauges are used selectively in different measurement applications.
In general, a Pirani vacuum gauge or other thermal conduction vacuum gauge, quartz friction vacuum gauge, or rotation-type viscosity vacuum gauge other vacuum gauge based on gas transport phenomena is used in high-pressure regions (low vacuum regions) from approximately 1 Pa to 105 Pa. For pressure measurements during processes, diaphragm-type vacuum gauges are primarily used in response to demands for ease of pressure control and high accuracy. On the other hand, in low-pressure ranges (high vacuum ranges) of 1 Pa or below, ionization vacuum gauges, of which the Bayard-Alpert ionization vacuum gauge (hereafter called xe2x80x9cB-A ionization vacuum gaugexe2x80x9d) is representative, are widely used. In addition, hybrid type vacuum gauges, which combine a vacuum gauge for measurements in high-pressure regions (low vacuum regions) with a vacuum gauge for measurements in low-pressure regions (high vacuum regions), have been developed as vacuum gauges to perform pressure measurements over a broad range extending from atmospheric pressure to high vacuum regions.
As an example of such a hybrid type vacuum gauge, Japanese Patent Application Laid-open No. 62-218834 discloses a vacuum gauge in which a quartz friction vacuum gauge used for measurements in high-pressure regions (low vacuum regions) from approximately 1 Pa to 105 Pa, and a B-A ionization vacuum gauge used for measurements in low-pressure regions (high vacuum regions) of 1 Pa or lower, are installed on the same flange.
The structure and principle of operation of this vacuum gauge are explained referring to FIG. 4 and FIG. 5. FIG. 4 is a cross-sectional diagram showing the structure of an ionization vacuum gauge of the prior art; FIG. 5 is a block diagram showing the control circuitry of an ionization vacuum gauge of the prior art.
This vacuum gauge installs a B-A ionization vacuum gauge and a quartz friction vacuum gauge on a common flange 2b, connected by an O-ring 5 to the vacuum vessel 1. The B-A ionization vacuum gauge part consists of three electrodes, namely an ion collector 8, filament 3, and grid 4; each is connected to a current introduction terminal mounted on the flange 2b. The installed quartz friction vacuum gauge consists of a quartz oscillator 18, connected to the current introduction terminal mounted on the flange 2b, and a quartz oscillator vessel 19.
Pressure regions of 10xe2x88x921 Pa or lower are measured using the B-A ionization vacuum gauge.
In low-pressure regions (high vacuum regions), when a positive grid voltage is applied to the grid 4 while simultaneously heating the filament 3 by passing a current, thermal electrons are emitted from the filament 3 toward the grid 4. Before arriving at the grid 4, these thermal electrons are accumulated within the grid 4 while undergoing oscillating motion in the vicinity of the grid, and collide with residual gas molecules within the vacuum vessel, which are ionized to create positive-charged ions.
When a thermal electron finally arrives at the grid 4, an emission current flows between the filament 3 and grid 4. On applying to the ion collector 8 a voltage (negative voltage) opposite the filament potential, positively-charged ions are captured by the ion collector 8, and consequently an ion current flows into the ion collector 8. At this time, if the voltages applied to each electrode are held constant and the emission current is fixed, then the density of thermal electrons undergoing oscillating motion in the vicinity of the grid 4 is constant. Hence the quantity of ions created is proportional to the concentration of gas molecules within the vacuum vessel 1 and therefore proportional to the pressure, so that by measuring the magnitude of the ion current flowing into the ion collector 8, the pressure within the vacuum vessel 1 can be measured.
On the other hand, pressures in the region from 1 Pa to atmospheric pressure are measured using a quartz friction vacuum gauge.
The oscillator vessel 19 is of a construction which envelops the quartz oscillator 18. Hence charged particles and thermal radiation emitted from the B-A ionization vacuum gauge are blocked, and adhesion of evaporated and sputtered material on the quartz oscillator 18 is prevented. The aperture part 50 exposes the quartz oscillator 18 to the gas pressure within the vacuum vessel 1.
When a constant AC voltage is applied to the quartz oscillator 18 to cause oscillation at a resonance frequency, the resistive component of the AC impedance changes with the gas pressure. Hence by measuring the resistive component of the AC impedance, the pressure within the vacuum vessel 1 can be measured.
Next, the operation of this vacuum gauge is explained, referring to FIG. 5.
The filament 3 of the B-A ionization vacuum gauge is connected to pins 12a and 12b, the ion collector 8 is connected to pin 9, and the grid 4 is connected to pin 10. The lead wires 21 of the quartz oscillator 18 are connected to pins 22a and 22b. A power supply for filament operation (abbreviated to xe2x80x9cFOPSxe2x80x9d) 27 is connected to the pins 12a and 12b via a filament shutoff switch (FSS) 39, to heat the filament 3 and cause emission of thermal electrons. When it is confirmed that the pressure within the vacuum vessel 1, measured by means of the quartz oscillator 18, has reached a prescribed pressure, the filament shutoff switch 39 is turned on, the power supply for filament operation 27 supplies a current to the filament 3, and the B-A ionization vacuum gauge is operated. The collector potential power supply (CPPS) 51 is connected to pin 9, and by holding the collector potential at, for example, xe2x88x9250 V, ions which have been generated are collected. An ion collector ammeter (ICA) 33 is connected between the collector potential power supply 51 and ground, to measure the ion current value. The ion current is converted into a pressure by the ion current-pressure conversion circuit (ICPCC) 34, and the result is displayed on a display device (DD) 25. The grid potential power supply (GPPS) 29 is connected to pin 10, to maintain a positive voltage (for example, +150 V) at the grid 4. As a result, thermal electrons emitted from the filament 3 can be captured.
A phase-locked loop (PLL) circuit 36 is connected between pins 22a and 22b, to cause stable oscillation of the quartz oscillator 18 at a characteristic frequency. A resonance voltage signal corresponding to the resonance impedance is converted into a pressure value by a resonance impedance-pressure conversion circuit (RIPCC) 37, and the result is displayed on the display device 25.
The control circuit (CC) 38 is connected to the resonance impedance-pressure conversion circuit 37; when it is detected that the pressure measured by the quartz oscillator 18 has fallen below a prescribed value (for example, 1 Pa), a control signal is sent to the filament shutoff switch 39 and to the pressure display device 25. As a result of toggling of the filament shutoff switch 39 by this control signal, the B-A ionization vacuum gauge operation is switched, and a wide range of pressures, from atmospheric pressure to high vacuum, is measured.
However, the conventional technology described above has problems such as the following.
As a first problem, by combining a B-A ionization vacuum gauge and a quartz friction vacuum gauge in the same flange, the flange dimensions become large compared with the flange dimensions when each individual measurement element alone is installed in the flange. In addition to the need to secure space for installation in the vacuum vessel, there are considerable constraints on the dimensions of the flange for installation.
As a second problem, if a quartz friction vacuum gauge is installed adjacent to a B-A ionization vacuum gauge, even if the quartz oscillator from the shield case by means of the solid-state thermal conduction of the oscillator lead wires, and consistently thermal effects on the quartz oscillator cannot be avoided. Inparticular, during a degassing operation by passing a current through the grid, the construction is such that the quartz oscillator fully feels the effects of thermal radiation from the grid. The resonance impedance of the quartz oscillator varies with temperature as well as with pressure. Hence when a current is passed through the filament and grid, or even when no current is passed but thermal effects remain due to residual heat, the value of the pressure measured by the quartz friction vacuum gauge contains, to some extent, a measurement error due to temperature fluctuations.
As a third problem, it is not rational, for purposes of avoiding the influence of gas-phase ions and secondary electrons arising from ionization of gas molecules, or of sputtered material, to install another pressure measurement element within the space for gas molecule ionization in a B-A ionization vacuum gauge. Gas-phase ions and secondary electrons, or sputtered material, are incident on the quartz oscillator via the aperture part of the shield, and a shock effect cannot be avoided.
As a fourth problem, by installing the quartz friction vacuum gauge adjacent to both the filament electrode and the grid electrode of the B-A ionization vacuum gauge, a new electric field due to the quartz friction vacuum gauge is created in the vicinity of the grid within the measurement element vessel. Hence the density of thermal electrons accumulated in the grid is reduced, and there is a major effect on the electric field contributing to the advance of thermal electrons which are to ionize the gas molecules. As a result, the measurement sensitivity of the B-A ionization vacuum gauge is degraded.
As a fifth problem, when, on installation of the measurement elements, the vacuum vessel wall behind the filament is distant, gas-phase ions flow to the filament, so that the actual emission current is reduced, and consequently the measurement limit on the high-pressure side of 10xe2x88x921 Pa or higher is degraded.
Problems such as these occur not only in the case of B-A ionization vacuum gauges, but in nearly all ionization vacuum gauges comprising as component electrodes a filament, grid, and ion collector. And, the above problems are not limited to a quartz friction vacuum gauge as a separate measurement element, but occur for any measurement element which is easily affected by heat, electrons, and so on from an ionization vacuum gauge.
Hence one object of this invention is to provide an ionization vacuum gauge which can stably perform measurements over a measurement region extending both from intermediate to high vacuum regions using a first ionization-type measurement part, and also to the measurement region intrinsic to a second measurement part.
Specifically, the above problem with increases in flange dimensions, and also the problems of the effect of thermal radiation from the filament and grid and of the effect of gas-phase ions, are resolved. Further, the problem of the occurrence of nonuniformity in the cylindrical electric field formed between the measurement element vessel and grid is resolved. And, the problem of gas-phase ions flowing into the filament, so that the actual emission current is reduced, is resolved.
In order to attain the above objects, the ionization vacuum gauge of this invention comprises a measurement element vessel, and first and second measurement parts provided within the measurement element vessel. The first measurement part comprises component electrodes including a filament, grid, and ion collector, and is of an ionization-type construction which measures the pressure of a vacuum state. The second measurement part has a construction with functions differing from those of the first measurement part. In this ionization vacuum gauge, the second measurement part is installed near the extension of the axis of the grid, in an area removed from the principle area of flight of thermal electrons emitted from the filament.
Here xe2x80x9cgrid axisxe2x80x9d signifies an axis in the interior of the coil shape of a grid wound into a coil shape, and which is essentially the axis of symmetry of the grid. xe2x80x9cNear the extension of the grid axisxe2x80x9d means on or near the extension of the grid axis. xe2x80x9cPrinciple area of flightxe2x80x9d refers to the area, among the areas through which thermal electrons fly, in which there is a relatively large amount of flight. xe2x80x9cArea removed from the principle area of flight of thermal electrons emitted from the filamentxe2x80x9d refers to an area in which shock effects due to thermal electrons, secondary electrons created or generated accompanying the ionization of gas molecules, and gas-phase ions, can be reduced as much as possible. To xe2x80x9chave functions differing from the first measurement partxe2x80x9d means that even if the second measurement part is a measurement part which measures pressure, the construction is different from that of the first measurement part, so that the principle of measurement is different, or the range of pressure measurement is different.
By adopting such a configuration, even if both a first and a second measurement part are incorporated in the measurement element vessel, the size of the measurement element vessel is not so large compared with a vacuum gauge having a single measurement part, and moreover there is no need to change the flange size, so that the first problem can be resolved.
Moreover, this configuration acts to minimize the effect on the second measurement part of thermal radiation from the filament and grid, so that the second problem can be resolved.
Further, thermal electrons accelerated between the filament and the grid and which accumulate in the grid vicinity primarily fly in directions perpendicular to the grid axis, so that this configuration acts such that the second measurement part does not receive the shock effects due to these thermal electrons, or due to secondary electrons generated accompanying the ionization of gas molecules, or due to gas-phase ions, so that the third problem can be resolved.
As a result, measurements can be performed stably over both intermediate to high vacuum regions by the first measurement part, employing an ionization-type measurement part, and also over the characteristic measurement region of the second measurement part.
In implementing this invention, preferably the second measurement part should be installed in the space between that wall of the measurement element vessel which is near the extension of the above-mentioned grid axis, and the component electrodes.
In implementing this invention, preferably a shield plate should be provided between the second measurement part and the component electrodes, to spatially separate the second measurement part and the above-mentioned component electrodes.
Here, xe2x80x9cspatially separatexe2x80x9d means to mostly separate spatially. If spaces are completely separated, the gas the vacuum of which is to be measured cannot enter into the second measurement part, and consequently accurate measurements cannot be performed by the second measurement part. By means of such a configuration, the second and third problems can be further resolved satisfactorily, and there is also the advantage that the effect on the second measurement part of the electric field formed by each of the component electrodes of the ionization-type first measurement part is reduced.
In implementing this invention, preferably the potential of the shield plate should be ground potential.
In this case, there is the further advantage that the effect of the electric field formed by each of the component electrodes of the ionization-type first measurement part on the second measurement part is further reduced.
In implementing this invention, preferably the measurement element vessel should have an approximately axially-symmetric shape, and the above-mentioned grid should be disposed such that the axes of the measurement element vessel and of the grid approximately coincide.
By thus positioning the grid in the approximate center of the measurement element vessel, the distance between the grid and the inner wall of the measurement element vessel can be maintained nearly constant. Hence the density of thermal electrons accumulating within the grid does not decrease, and no large effect is exerted on the electric field contributing to the advance of thermal electrons to cause ionization of gas molecules. As a result, pressure can be measured with stable sensitivity by the first measurement part, and hence the fourth problem can be resolved.
In implementing this invention, preferably the potential of the measurement element vessel should be lower than the filament potential.
By adopting such a configuration, the wall of the measurement element vessel plays the role of an auxiliary electrode which prevents the flow of gas-phase ions into the filament to reduce the actual emission current. Hence the measurement limit is extended to the high-pressure region of 10 Pa. Consequently, the fifth problem can be resolved.
In implementing this invention, preferably the second measurement part should be fixed to an element fastening plate consisting of material with excellent thermal conductivity.
By fixing the second measurement part to an element fastening plate, the effect of vibrations from outside can be reduced.
Further, the element fastening plate consists of material with excellent thermal conductivity, so that even if thermal radiation is incident on the second measurement part, heat can escape from the second measurement part to the element fastening plate via the fixed part of the element fastening plate by means of solid-state thermal conduction.
In implementing this invention, preferably the element fastening plate should be made from a material with excellent thermal conductivity, and be connected to one end of a pipe, the other end of which protrudes outside the measurement element vessel, and the pipe preferably should be held at a low potential approximately equal to that of the measurement element vessel.
Here the pipe need not have an ordinary cylindrical hollow shape; so long as it is connected to the element fastening plate and of such a structure as to protrude out from the measurement element vessel, any shape is possible. Here, the second measurement part is connected to an element fastening plate consisting of material with excellent thermal conductivity and, via the pipe, with the measurement element vessel exterior. Hence even if thermal radiation is incident on the second measurement part, heat can be allowed to escape appropriately from the second measurement part to the measurement element vessel exterior via the fixed part of the element fastening plate, by means of solid-state thermal conduction.
In implementing this invention, preferably the measurement element vessel should have a coupling part that can be coupled removably to the vacuum vessel.
Here xe2x80x9cvacuum vesselxe2x80x9d means the part of the vacuum gauge other than the measurement element vessel wherein measurement parts to measure vacuum are installed; for example, a vacuum chamber or similar.
By means of such a configuration, the operation of installation of the measurement element vessel in the vacuum vessel can be improved, and at the same time, the measurement element vessel installation space and installation cost can be held to a minimum.
In implementing this invention, preferably, the measurement element vessel should comprise, in part of a wall of the measurement element vessel, a current introduction terminal made from an insulating material for applying specified voltages to the component electrodes of the first measurement part and the second measurement part and for fixing in place the component electrodes; and the current introduction terminal should be provided with pins to introduce current to the first measurement part and the second measurement part.
In implementing this invention, preferably the ionization vacuum gauge should comprise a control circuit to operate the first and second measurement parts.
In implementing this invention, preferably the control circuit should comprise a degassing power supply and degassing switch, in order to remove gas adhering to the measurement element vessel.
Here, to xe2x80x9cremove gas adhering to the measurement element vesselxe2x80x9d means to remove gas molecules adsorbed by the inner wall surface of the measurement element vessel, component electrodes and other elements, and which may become causes of error during measurements by measurement parts. By comprising a degassing power supply and degassing switch, adsorbed molecules can be removed, and measurement errors can be prevented.
In implementing this invention, the first measurement part may be a Bayard-Alpert type ionization vacuum gauge.
In implementing this invention, the second measurement part may be a measurement part which measures pressure.
In implementing this invention, the measurement part which measures pressure may be a quartz oscillator type pressure gauge.
In implementing this invention, the second measurement part may be a measurement part which measures temperature.